For an idler bearing seat produced traditionally by means of a method of interference fit, it is necessary to machine stepped holes at both ends of an idler steel tube and then press and fit the bearing seat therein, no matter that the bearing seat is made of cast iron, punched cold-rolled steel sheet or engineering plastic. FIG. 1 shows an idler comprising a bearing seat of cast iron, FIG. 2 shows an idler comprising a bearing seat of punched cold-rolled steel sheet, and FIG. 3 shows an idler comprising a bearing seat of engineering plastic. In FIGS. 1-3, the contact length between the bearing seat and the stepped hole of the idler steel tube is 15-20 mm. FIG. 4 shows an idler produced by welding a punched bearing seat and the idler steel tube. In FIG. 4, the contact length between the punched bearing seat and the stepped hole of the idler steel tube is 2-5 mm (the thickness of the punched sheet steel). Because the idler steel tube itself has certain non-roundness, it is difficult to ensure the concentricity of the stepped holes on both ends of the idler tube during machining. Even if a special tool is employed to fix and machine the stepped holes on both ends of the idler tube in one time, it is also difficult to eliminate the non-roundness of the outer diameter of the tube. At the same time, the production cost will increase. As for producing an idler by the method of welding, since it is prone to generate an error during positioning, and in particular, welding deformation cannot be controlled, it is also difficult to ensure the concentricity of the bearing seats on both ends of the idler tube, which makes an amplitude of a fluctuation curve of the rotation resistance of the idler very great. An idler has been disclosed that is produced by the interference fit between a bearing seat of engineering plastic and an idler tube without stepped holes at both ends thereof. Although this method can ensure the concentricity of the bearing seats on both ends of the idler tube and eliminate most of the non-roundness of the tube, the wall thickness of the idler steel tube is small (1.5-3.5 mm), which can meet only light load applications. So this method is unsuitable for producing an idler capable of bearing medium- or heavy-load by using a thick-walled steel tube, and cannot meet the usage requirements in many situations.
Additionally, for the seals on both ends of the prior art idlers, different forms of labyrinth structures are employed mostly. However, whatever labyrinth seal is used, it cannot attain the object of well preventing trickling water, dust and mud from contaminating the idler bearing, resulting in premature failure of many idlers, the normal operation of a belt conveyor being influenced, a greater drive power being required, and a waste of the energy source and a serious wear of the rubber belt. For some other idlers employing a contact-type seal, the rotation resistance of the idler with such contact-type seal is large. Moreover, if the trickling water is combined with the wear of impurities such as coal dust and silt, the working period will also be very short.